CN1177240A - Optical communication system - Google Patents

Abstract

When the branch unit combines the first optical signal and the second optical signal sent by the branch station in the optical add-drop system, the second optical signal is different from the first optical signal in terms of power and light level, and is transmitted from the terminal station A or terminal The signal-to-noise ratio (S/N) of the lower power light level of the two different power light levels sent by station B is smaller, thereby deteriorating the system performance. Therefore, the analog light is sent together with the optical signal to adjust the power level of the optical signal. Alternatively, an optical attenuator or an active optical signal light level adjustment unit is placed in the branch unit, so that the light levels of the two optical signals to be combined can be equal.

Description

optical communication system

The invention relates to an optical communication system applied to long-distance communication, such as submarine optical cable communication.

Recently, the development of optical communication systems has been widely carried out to realize large-capacity and high-speed communication systems. In particular, when a large amount of information is to be transmitted simultaneously, an optical wavelength multiplexing system is highly evaluated, and research is being conducted for practical use in the near future. In the optical wavelength multiplexing system, the optical signal carrying information and containing multiple wavelengths is used for wavelength multiplexing transmission. Optical signals of each wavelength correspond to at least one channel. In an optical wavelength multiplexing system applicable to long-distance communication, such as submarine optical cable communication, an optical add-drop system is being developed. In this system, among a plurality of optical signals wavelength-multiplexed on a communication line, An optical signal with a specific wavelength, or an optical signal transmitted along a specific channel, is branched and transmitted along the branch channel to the terminal station, and the optical signal has the same wavelength as the branch channel and is sent from the terminal station , and then combined into the optical signal transmitted through the original transmission line and transmitted to the terminal station.

Figures 1A to 1F illustrate conventional light add-drop systems and the problems associated with such systems.

Fig. 1A is a block diagram showing the entire configuration of this light adding-dropping system. The basic configuration in this optical add-out system includes: terminal station A as a transmitting station for transmitting optical wavelength multiplexed optical signals; terminal station C as a receiving station for receiving signals from terminal station A; A branching unit 1100 into which an optical signal having a specific wavelength is branched or combined in an optical signal; and a terminal station B which receives the optical signal branched by the branching unit 1100 and forms an optical signal having the same wavelength as the received optical signal Send a new message. Usually in submarine optical cable communication, the branch unit 1100 is fixed on the seabed, and sends optical signals to terminal stations installed in different countries, for example, terminal station A, terminal station B and terminal station C. Generally, the distance between the terminal station A and the terminal station C is about 3,000 km, and the branch unit 1100 is installed near the midpoint between these two stations. When an optical signal is transmitted over a long distance, since the strength of the optical signal is attenuated, there are There are a plurality of optical amplifiers 1101, 1102 and 1103 on the transmission lines respectively. FIG. 1A draws respective optical amplifiers 1101, 1102 and 1103 on corresponding transmission lines for simplicity of illustration, however, each transmission line actually has many optical amplifiers. Generally, each optical amplifier 1101, 1102, and 1103 has an automatic output light level (level) control circuit (ALC circuit), to keep the output light level of each optical amplifier 1101, 1102, and 1103 constant, so that the optical signal can be constantly amplified to Specific output light level.

Figure 1A shows a transmission line for unidirectional communication. In fact, the circuit is designed to establish two-way communication, ie uplink communication and downlink communication.

1B to 1F illustrate an optical signal on each transmission line and its problems.

Figure 1B plots the optical signal at point A in Figure 1A. In the case shown in FIG. 1B, an optical signal having four different wavelengths is wavelength-multiplexed and transmitted from a terminal station A. As shown in FIG. The bump under each optical signal is called amplified spontaneous emission (ASE) noise. This is generated when the noise superimposed on the optical signal is amplified by the optical amplifier. The operating performance of an optical communication system depends on the signal-to-noise ratio S/N between the optical signal and ASE.

In the branching unit 1100, the optical signal with the wavelength _λ1 is split and transmitted to the terminal station B, and the optical signal with the wavelength _λ1 is transmitted from the terminal station B to the terminal station C.

Among the signals transmitted from terminal station A (FIG. 1B), optical signals with a wavelength other than _λ1 are not split by the branching unit 1100, but are transmitted to terminal station C as they are. Terminal station B receives an optical signal with wavelength _λ1 , and transmits an optical signal with the same wavelength _λ1 . FIG. 1C shows the state at point B of a signal transmitted from terminal station B and amplified by optical amplifier 1102 . The branching unit 1100 combines the optical signal with the wavelength _λ1 transmitted from the terminal station B with the light with the wavelengths _λ2 to _λ4 , and transmits the combined result to the terminal station C.

FIG. 1D shows the state at point C of the optical signal from the terminal station B, which is combined by the branching unit 1100 and amplified by the optical amplifier 1103 . FIG. 1C and FIG. 1D show that when the optical signal from the terminal station B is combined with the optical signal from the terminal station A, the power levels of the optical signals are equal to each other. In this case, an optical signal having any wavelength exhibits the same S/N ratio to ASE noise, as shown in FIG. 1D .

Figure 1E also depicts the state of the optical signal at point B. In this case, the power level of the optical signal from the terminal station B is high. When the power of the optical signal from terminal station B is high, the optical signal is combined by the branching unit 1100 and amplified by the optical amplifier 1103, and the state of the optical signal at point C becomes as shown in FIG. 1F. Therefore, although the signal-to-noise ratio S/N of the wavelength _λ1 is high, since the operation performance of the optical communication system depends on the lower signal-to-noise ratio S/N, when the signal-to-noise ratio S/N of other wavelengths is low, it is considered The system performed poorly.

2A, 2B, 3A, and 3B show the operation of the optical amplifier and the signal-to-noise ratio S/N.

In this example, two optical signals having different wavelengths are multiplexed, and an optical signal with a total power of 0 dBm is input to an optical amplifier. The optical amplifier includes an automatic output light level control circuit with a gain of 10dB, and the light output is limited to 10dBm. The state of each optical signal at the input end is -3dBm for the powers of the optical signals of the two wavelengths, and the sum is 0dBm, as shown in FIG. 2A . FIG. 2B shows the output of these two optical signals after they are input to the optical amplifier. That is, the optical signal of each wavelength is amplified, the power of each optical signal is +7dBm, and the total power of the output light is +10dBm. On the other hand, the ASE noise is also amplified, and the signal-to-noise ratio S/N of each optical signal to the ASE noise is 30dB. Therefore, the operation performance of the optical amplifier shows a signal-to-noise ratio S/N of 30 dB.

3A and 3B illustrate the multiplexing of an input optical signal with another optical signal having a different power level. The performance of the optical amplifier is the same as that of the optical amplifier shown in Fig. 2A and Fig. 2B. However, as shown in FIG. 3A , the total power of two optical signals with different wavelengths is 0 dBm, the power level of one optical signal indicates -1.5 dBm, and the power level of the other optical signal indicates -4.5 dBm. The difference between the two power levels is 3dB. If these two optical signals are taken as input, the resulting output is shown in Figure 3B. That is, the higher optical signal power level of the two input signals is +8.5dBm, while the lower optical signal power level is +5.5dBm, because the optical signal of each wavelength is amplified, so that the total power level of the output signal can be Taking the above value, that is, the output of the optical amplifier is fixed at +10dBm. At this time, the ASE noise is amplified, and the S/N ratios of the two wavelengths are different. That is, the signal-to-noise ratio S/N of the wavelength indicating the higher power light level is an acceptable value, while the signal-to-noise ratio S/N of the wavelength indicating the lower power light level is relatively unsatisfactory. Since the operational performance of the optical amplifier was evaluated by an unsatisfactory signal-to-noise ratio S/N, the performance of the optical amplifier was judged to be poor.

In the optical add-drop system described above with reference to FIG. 1A, many optical amplifiers are inserted between the terminal station and the branch unit. In the branch unit, an optical signal independently generated from terminal station A is combined with an optical signal from terminal station B, and amplified by an optical amplifier. The optical signals of corresponding wavelengths from end station A and end station B may not match in power when combined because their transmission distances and output powers are not the same. In addition, even if the output power and attenuation of the optical signal have been carefully calculated in the design stage of the system, the power level of the optical signal cannot be controlled as designed. In this case, the optical signal with a lower power level and the optical signal with a higher power level, after being amplified by the optical amplifier, have a difference in the signal-to-noise ratio S/N, as shown in Figures 2A, 2B, 3A and 3B as described. The operational performance of the system is evaluated by the signal-to-noise ratio S/N of the flat optical signal having a lower power, ie, by an unsatisfactory signal-to-noise ratio S/N.

When the power level of the optical signal from the branch station is different from the power level of the optical signal from the sending station, the evaluation is based on the signal-to-noise ratio (S/N) indicating that the optical signal transmission performance is low, so that the system performance is determined to be poor.

SUMMARY OF THE INVENTION An object of the present invention is to provide an optical communication system capable of compensating for the difference between the power level of an optical signal from a transmitting station and that from a branch station, thereby maintaining high system performance.

The optical communication system according to the present invention includes: a transmitting station for transmitting a wavelength multiplexed optical signal; a receiving station for receiving the optical signal; receiving an optical signal having a specific wavelength in the wavelength multiplexed optical signal, and A branch station that sends an optical signal; and a branch unit, which separates the optical signal with a specific wavelength from the optical signal sent by the sending station, transmits it to the branch station, and makes the optical signal sent by the branch station different from the one sent by the branch station. Optical signals of wavelengths are combined together. These signals are composed of signals whose power and light levels match each other.

In an optical communication system, it includes a sending station that sends a wavelength multiplexed optical signal; a receiving station that receives the optical signal; receives an optical signal with a specific wavelength in the wavelength multiplexed optical signal, and sends the specific wavelength The branch station of the optical signal; and the branch unit, which separates the optical signal with a specific wavelength from the optical signal sent by the sending station, transmits it to the branch station, and combines the optical signal sent by the branch station with the optical signal sent by the sending station; Send the combined signal to the receiving station, according to the branch unit of the present invention, it will separate the optical signal with a specific wavelength from the optical signal sent by the sending station, transmit it to the branch station, and combine the optical signal sent by the branch station with the branch station Light signals emitted at different wavelengths are combined together. These signals are composed of signals whose power and light levels match each other.

In addition, a terminal station according to another aspect of the present invention includes an optical transmission signal transmitting unit for generating an optical transmission signal modulated by data to be transmitted; an analog light (dummy light) generating unit for generating analog light; a wavelength multiplexing unit for wavelength multiplexing of analog light and optical transmission signals; and a light level adjustment unit for adjusting the output light level of the analog light.

According to another aspect of the present invention, there is a system in the method for controlling the optical communication system, it comprises a first optical terminal station; A second optical terminal station; A third optical terminal station; An optical branching unit for connecting the first optical terminal station to a third optical terminal station; and an optical amplifier to keep the output signal between the optical branching unit and the second optical terminal station at a constant power, wherein the branching unit is to the light from the first terminal station and the second terminal station The transmission signal is wavelength multiplexed and the result is sent to the third terminal station, by sending the analog light with a wavelength different from the optical transmission signal and adjusting the optical level of the analog light, the second optical terminal station controls the optical amplifier to output the optical transmission signal of the light Guangping.

In addition, the terminal station in the optical communication system according to the present invention has a system comprising a first optical terminal station; a second optical terminal station; a third optical terminal station; an optical branching unit for connecting the first optical terminal station to a third optical terminal station; and an optical amplifier for maintaining an output signal between the optical branching unit and the second optical terminal station at a constant power, wherein the branching unit transmits optical signals from the first terminal station and the second terminal station wavelength multiplexing and sending the result to a third terminal station, the second optical terminal station includes an optical transmission signal transmission unit for generating an optical transmission signal modulated by data to be transmitted; an analog light generation unit for generating analog light different in wavelength from the optical transmission signal; a wavelength multiplexing unit for wavelength multiplexing the analog light and the optical transmission signal; and an optical level adjustment unit for adjusting the output optical level of the analog light.

In the terminal station or the branch unit of the optical communication system according to the present invention, when the optical signal sent by the sending station that transmits the wavelength multiplexed optical signal, but does not include the optical signal with a specific wavelength to be sent to the branch station, in the branch unit When combined with an optical signal with a specific wavelength sent by a branch station, this combination can be realized by two optical signals whose powers match each other. Therefore, the difference in power level between the two optical signals after combination avoids a decrease in the signal-to-noise ratio S/N of the lower power level signal and avoids degradation of the system performance. That is, the present invention can realize a light add-drop system which can keep system performance at a high level for a long time.

Figures 1A to 1F are diagrams illustrating common insertion-extraction systems and their problems;

2A and 2B are explanatory diagrams (1) of the operation of the optical amplifier and its signal-to-noise ratio S/N;

3A and 3B are explanatory diagrams (2) of the operation of the optical amplifier and its signal-to-noise ratio S/N;

Fig. 4 represents the first embodiment of the present invention;

Fig. 5 shows an example of an optical attenuator according to the first embodiment;

Fig. 6 has drawn the configuration according to the second embodiment of the present invention;

Fig. 7 draws the overall configuration according to the third embodiment of the present invention;

Fig. 8 shows the configuration of adding and taking out optical signals in the branching unit according to the third embodiment of the present invention;

Fig. 9 A to Fig. 9 C draw the characteristic curve of branch unit shown in Fig. 8;

10A and FIG. 10B represent the state of the added optical signal input to the branching unit in which the added optical signal is combined, and show the output light from the branching unit to the receiving station;

Fig. 11A and Fig. 11B represent the situation that simulated light is used as the control means according to the third embodiment of the present invention;

FIG. 12 is a block diagram showing a partial configuration of a terminal station as a transmitting station and a receiving station; and

Fig. 13 is a block diagram showing the configuration of a branch station.

Figure 4 shows a first embodiment according to the invention.

Figure 4 depicts this configuration in which the branching unit 16 adjusts the power level of the optical signal from the branching station. Although only the downlink from the sending station to the receiving station is shown in FIG. 4, an uplink from the receiving station to the sending station is actually installed.

The optical signal sent from the sending station is wavelength multiplexed, and this optical signal is input to the circulator 10, and then reaches the fiber grating 11. In the fiber grating 11, only the optical signal with the wavelength to be sent to the branch station is reflected, and other signals pass through directly. The optical signal reflected by the fiber grating 11 is input to the circulator 10 again, and sent to the branch station. The optical signal directly passing through the FBG 11 also passes through the isolator 12 and the FBG 13, enters the circulator 14, and is combined with the optical signal from the branch station, and then sent to the receiving station. When the optical signal from the branch station is input to the circulator 14, the signal is transmitted to the fiber grating 13. Because the wavelength of the optical signal sent by the branch station is the same as the wavelength reflected by the fiber Bragg grating 11, this signal is also reflected by the fiber Bragg grating 13, and then input to the circulator 14, and sent to the side of the receiving station. The excess light that is not reflected by the fiber grating 13 is blocked by the isolator 12 to prevent it from propagating to the sending station. When the optical signal sent by the sending station and the optical signal sent by the branch station are combined in the circulator 14, if there is a difference in the power of the optical signals with different wavelengths, after the optical amplifier amplifies the signal, the signal-to-noise ratio S/N will be decline. According to the case of this embodiment, there is an optical attenuator 15 on the transmission line, and the optical signal sent from the branch station passes through this attenuator. The optical attenuator 15 adjusts the power light level of the optical signal from the branch station so that the optical signal sent from the sending station matches the optical signal sent from the branch station in terms of power light level. Therefore, the system performance of the optical add-drop system can be kept high.

Fig. 5 shows an example of an optical attenuator in the first embodiment according to the present invention.

The optical attenuator shown in FIG. 5 is obtained by fusion-splicing single-mode fibers or dispersion-shifted fibers (DSF) 20 and 21 with their optical axes shifted from each other. Single mode optical fiber or DSF 20 or 21 includes cores 22 and 24 and claddings 23 and 25 which protect cores 22 and 24 . They are melted in melting zone 26 . There is some displacement between core 22 and core 24 relative to each other, and when the optical signal passes through this area, there is optical loss. Therefore, the light level of the optical signal power passing through this region is lower than the light level of the optical signal power before the optical signal passes through this region. Therefore, the power level of the optical signal sent from the branch station can be adjusted. The connection between such fibers is called axis-shift splicing.

When the optical attenuator is made by the axis-shift splicing method, the connection between the optical fibers is fixed, so the attenuation of the optical signal is also fixed. Therefore, when the system is designed, the attenuation of the optical signal can be adjusted only once with the axis-shift splicing. However, since the optical attenuation can be maintained at a constant level for a long time, a reliable optical attenuator can be obtained in the case of a submarine branch unit installed in submarine optical cable communication, and where the branch unit cannot be frequently maintained.

Because the unit that completes the splicing process often contains equipment to detect optical signal attenuation, the optical attenuation is usually adjusted by adjusting the displacement of the optical axis of the optical fiber, and the optical attenuation is determined when the optical fiber is spliced. Therefore, suitable light attenuation can be realized.

The structure of the optical attenuator is not limited to the above-mentioned axis-shift splicing, but can be freely determined by ordinary professional technicians within the expected range.

Fig. 6 shows the configuration of the second embodiment of the present invention.

Likewise, in this embodiment, the branch unit 30 is designed to adjust the power level of the optical signal sent by the branch station to match the power level of the optical signal sent by the sending station. In Fig. 3, only the downlink from the sending station is indicated. (Actually, there can be uplinks.)

The optical signal sent from the transmitting station is amplified by the optical amplifier 31 and branched by the coupler 32 . Because the branching process implemented in this example is used to monitor the optical signal power level from the sending station, most of the optical signal power is designed not to be branched but to pass through directly. From the directly passing optical signals, optical signals of those wavelengths to be transmitted into the circulator 33 and the fiber grating 34 and to the branch station are extracted, and the extracted optical signals are sent to the branch station. Optical signals with other wavelengths also directly pass through the isolator 35, fiber grating 36 and circulator 37, and are combined with the optical signals from the branch station, and the combined result is sent to the receiving station.

The optical signal split from the coupler 32 is converted into an electrical signal by the photodiode 38 in the control circuit 400 and input to the comparator 39 . The optical signal sent from the branch station is input to the optical amplifier 43 for amplification, and then branched by the coupler 44 . At this point, most of the optical signal passes through directly and is combined with the optical signal directly passed from the sending station in the circulator 37 and the fiber grating 36, and the result of the combination is sent to the receiving station side. The optical signal split by the coupler 44 is converted into an electrical signal by the photodiode 41 in the control circuit 400 . The power level of the signal converted into an electric signal and received by the light level converter 40 is adjusted. input to comparator 39 . The reason for configuring the optical level converter 40 is as follows. That is, the optical signal received by the photodiode 38 is, for example, optical signals of 8 different wavelengths emitted from the transmitting station and multiplexed, however, the optical signal received by the photodiode 41 is emitted from the branch station and includes Optical signals of, for example, 4 wavelengths among 8 different wavelengths among the optical signals transmitted from the transmitting station. Therefore, the light signal received by photodiode 38 contains 8 light signals, while the light signal received by photodiode 41 contains only 4 light signals. If the power levels of these optical signals are directly compared, the power level of the optical signal received by the photodiode 38 is naturally higher. However, it is necessary to match the power level of the optical signal per wavelength emitted from the branch station with the power level of the optical signal per wavelength emitted from the transmitting station but not taken out (extracted) to the branch station. Therefore, the power level of the four-wave multiplexed optical signal from the branch station is converted by the optical level converter 40 to match the power level of the eight-wavelength multiplexed optical signal from the sending station. Then, the converted result is input to the comparator 39 .

The comparator 39 compares the electric signal power levels thus obtained, and the comparison result is input to the operational amplifier 42 . The comparison result is compared with the reference value (ref), and if there is a difference between the optical signal power level from the branch station and the optical signal power level from the sending station, a control signal is sent to the optical amplifier 43 so that the optical signal from the branch station The signal power level can be adjusted to match the power level of the optical signal of each wavelength directly passing through the sending station with the power level of the optical signal of each wavelength output by the optical amplifier 43 .

Therefore, when the optical signals are combined by the circulator 37 and the fiber grating 36, the power levels of the two optical signals can be equalized. Therefore, the above-mentioned system performance degradation caused by the optical amplifier can be successfully avoided while the optical signal is being transmitted to the receiving terminal.

The configuration of the above-mentioned branch units is only an example, and there may be many changes, and these changes are covered by the process principle of this embodiment.

Fig. 7 shows a third embodiment of the present invention.

According to the first embodiment and the second embodiment, the difference in power level between the optical transmission signal passing through the branching unit and the optical signal inserted from the branching station is adjusted in the branching unit. On the other hand, according to the third embodiment, before the optical signal is input to the branching unit, the power level of the optical signal is adjusted under the control of the terminal station side.

Actually, the analog light having a wavelength different from that of the optical transmission signal is transmitted, and the power level of the transmission signal is adjusted in the optical terminal station by changing the power level of the analog light.

That is, when the light level of the analog light is raised, the light level of the light transmission signal is lowered when passing through the optical amplifier. By lowering the light level of the simulated light, the light level of the light transmission signal increases as it passes through the optical amplifier.

Fig. 7 draws the configuration of this system, and it comprises the optical transmission signal sending unit 1-1 at the optical terminal station 1; The analog light generation unit 1-2 is used to generate analog light at the optical terminal station; The light level adjustment unit 1-3 , for adjusting the optical level of the analog light; and wavelength multiplexing units 1-4, for combining optical signals of different wavelengths together. The light level of the optical transmission signal is adjusted by virtue of this configuration.

Fig. 8 shows a third embodiment of the present invention.

The configuration of this system is as shown in Figure 7, and it includes optical terminal station 1; Optical transmission signal sending unit 1-1; Analog light generation unit 1-2, is used to produce analog light at optical terminal station; Light level adjustment unit 1- 3, for adjusting the optical level of the analog light; and wavelength multiplexing units 1-4, for combining optical signals of different wavelengths. The light level of the optical transmission signal is adjusted by virtue of this configuration.

When the signal is sent to the optical terminal station 2 through the optical amplifier 6 , the signal from the optical terminal station 1 is amplified by the optical amplifier 6 . Therefore, the light level of the optical signal can be changed according to the light level of the analog light.

When the light level of the analog light is raised, the light level of the light transmission signal is lowered when passing through the optical amplifier 6 . By lowering the light level of the analog light, the light level of the light transmission signal increases when passing through the optical amplifier 6 .

That is, when optical signals of various wavelengths whose power levels are different from each other are input to the optical amplifier, the output of the optical amplifier can be set to a constant value as mentioned in the description of the prior art. Therefore, after each optical signal is amplified, its power light level is different. When each signal is input to the optical amplifier, one of the signals has a high power light level, while the other signal has a low power light level. For this reason, when an optical signal is transmitted from the branch station, the output power level can be varied and analog light having a wavelength different from that of the optical signal is transmitted together with the optical signal containing data information. Therefore, when the optical signal passes through the optical amplifiers 60-1 to 60-n and 61-1 to 61-n, the power level of the optical signal can be adjusted.

Fig. 7 shows the entire configuration according to the third embodiment of the present invention.

FIG. 7 shows a configuration of an optical add-drop system in which a terminal station A and a terminal station B are connected to each other through a branch unit 51 using an up line and a down line. Moreover, a line is branched from the branch unit 51, and the uplink and downlink are arranged so that the branch station 53 can send and receive optical signals. There are a plurality of optical amplifiers 55-1 to 55-n, 56-1 to 56-n, 57-1 to 57-n, 58- 1 to 58-n, 59-1 to 59-n, 60-1 to 60-n, 61-1 to 61-n, and 62-1 to 62-n, each of which has an ALC circuit when the optical signal ALCs amplify the light signal when transmitting over long distances. Branch unit 51 has an uplink and a downlink. The uplink includes an optical circulator 33, which is used to input the optical transmission signal from the optical terminal station A to the optical fiber grating 34, and transmits the optical transmission signal with a specific wavelength from the optical fiber grating 34 to the branch station 53; an optical isolator 35 , used to let the light that has passed through the fiber Bragg grating 34 pass; Output to the side of optical terminal station B. The downlink includes an optical circulator 33', which is used to input the optical transmission signal from the optical terminal station B to the optical fiber grating 34', and transmits the optical transmission signal with a specific wavelength from the optical fiber grating 34' to the branch station 53'; An optical isolator 35' is used to allow the light that has passed through the fiber grating 34' to pass through; and an optical circulator 37' is used to input the optical signal from the branch station 53' to the fiber grating 36' to optically isolate The light reflected by the fiber grating 35' and the fiber grating 36' is output to the side of the optical terminal station B.

Branch station 53 comprises receiving unit 1-6, is used for receiving the optical transmission signal from downlink; Optical transmission signal sending unit 1-1, is used for sending optical transmission signal; After the signal of the unit 1-6, for changing the light level of the analog light; and the wavelength multiplexing unit 1-4, for wavelength multiplexing from the analog light generating unit 1-2 and the optical transmission signal sending unit 1-1 The output of the output; the receiving unit 1-6' is used to receive the optical transmission signal from the uplink; the optical transmission signal sending unit 1-1' is used to send the optical transmission signal; the analog light generation unit 1-2' is used to receive After the signal from the receiving unit 1-6', it is used to change the light level of the analog light; and the wavelength multiplexing unit 1-4' is used for wavelength multiplexing from the analog light generating unit 1-2' and the optical transmission signal Output of sending unit 1-1'.

The adjustment between the optical signal power level from the branch station 53 and the optical signal power level from the terminal station A or terminal station B is to utilize the analog light generating units 1-2 and 1-2' in the branch station 53 and the terminal stations A and The optical spectrum analyzers 65 and 66 in the terminal station B are completed. The analog light generated by the analog light generating units 1-2 and 1-2' should be different in wavelength from the optical signal.

That is, when the analog light and the optical signal from the branch station 53 are multiplexed and transmitted together, after passing through the optical amplifier, the output of the optical signal can be adjusted according to the power level of the analog light. For example, when the power level of the optical signal is higher than that of the analog light, the output of the optical signal after being amplified by the optical amplifier is greater than the output of the analog light. On the other hand, when the power level of the analog light is higher than that of the optical signal, the output of the analog light is greater than the output of the optical signal after being amplified by the optical amplifier, and the optical signal has a lower power level. Since the output of the optical signal remains constant, the sum of the output power of the analog light and the output power of the optical signal should be a constant value. Therefore, changing the power level of the analog light also changes the power level of the optical signal output from the optical amplifier.

There are spectrum analyzers 65 and 66 in the terminal station A and terminal station B receiving the optical signal, which are used to detect the signal power level of a specific wavelength in the received optical signal. Whether there is a difference in the power level can be determined by detecting the power level of the optical signal of each wavelength transmitted from the branch station 53 and the power level of the optical signal directly transmitted from the terminal station A and the terminal station B. The result is transmitted to the branch station 53 as an optical signal. If the receiving unit of the branch station 53 confirms that the optical signal sent from the branch station is different in power light level from the optical signal sent directly from the terminal station A or terminal station B, the analog light generating units 1-2 and 1-2' are adjusted The analog optical power level of , so that the power level of the optical signal sent from the branch station 53 and output by the optical amplifier can be adjusted. Therefore, the optical signal power level of each wavelength is constantly monitored by the receiving terminal station, and the power level of the analog light is adjusted through the branch station 53, so that the optical signal power sent from the branch station 53 is the same as that directly sent from the terminal station A or terminal station B. When the optical signal power level of the optical signal is combined in the branch unit 51, the two are approximately equal. Therefore, high performance of the system can be maintained without destroying the operating characteristics of the system. Because only when the optical signal in the optical multiplexing wavelength is low-power light, the reduced signal-to-noise ratio S/N is obtained.

Fig. 8 shows the configuration of adding and taking out optical signals in the branching unit according to the third embodiment of the present invention.

Various elements of the uplink are omitted in FIG. 8 . The branching unit according to the third embodiment only performs the add-drop function of optical signals. That is, the wavelength-multiplexed optical signal emitted from the transmission station passes through the circulator 70 and is input to the fiber gratings 73-1 to 73-4. Each of the fiber gratings 73-1 to 73-4 functions to reflect an optical signal of a single wavelength. That is, the fiber gratings 73-1, 73-2, 73-3, and 73-4 selectively reflect optical signals of wavelengths λ ₁ , λ ₂ , λ ₃ , and λ ₄ emitted from the sending station, respectively, and then The corresponding optical signal is input to the circulator 70 . The optical signals reflected from the fiber gratings 73-1 to 73-4 enter the circulator 70 again, take different paths, and send them to the branch station as extracted optical signals. The optical signals reflected by the fiber gratings 73-1 to 73-4 pass through the isolator 72 and the fiber gratings 74-1 to 74-4, enter the circulator 71, combine with the added optical signals sent from the branch station, and then transmit to receiving station.

Added light signals and analog light emitted from the branch station are input to the circulator 71 and transmitted to the fiber gratings 74-1 to 74-4. As described above, the optical signals having the wavelengths _λ1 to _λ4 are reflected, input to the circulator 71, and transmitted to the receiving station. At this time, the analog light transmitted together with the optical signal as the addition signal is neither reflected by the fiber gratings 74 - 1 to 74 - 4 nor passes through the isolator 72 . Therefore, most of the signal is scattered. By virtue of this configuration, analog light is not transmitted to the side of the receiving station.

9A to 9C show characteristic curves of the branching unit shown in FIG. 8 .

Fig. 9A shows the characteristic curve of the path from the sending station to the receiving station. The incident light from the sending station is white light, and Figure 9A indicates the transmission characteristics around the isolator 72 (Figure 8). Figure 9A indicates that the intensity of light transmission around the four central wavelengths decreases. This means that the fiber gratings 73 - 1 to 73 - 4 reflect light of these wavelengths, which is not output to the isolator 72 . Wavelengths other than a specific wavelength keep their light intensity constant. Therefore, depending on the configuration shown in FIG. 8, optical signals with specific wavelengths can be selectively rejected.

Fig. 9B shows a characteristic curve of taking out an optical signal from a sending station to a branch station. The light from the sending station is white light. Figure 9B indicates that light at the low transfer coefficient wavelengths shown in Figure 9A is instead extracted and transmitted to the branch station. The lights of four different wavelengths are reflected by the fiber gratings 73-1 to 73-4 shown in FIG. 8, and transmitted to the branch station by the circulator 70.

FIG. 9C shows the transmission characteristic curve of the optical signal from the branch station to the receiving station. In this case, no light is input from the sending station, and the white light input from the branch station is to check which wavelength is to be detected. In this case, the light input from the branch station is sent to the fiber gratings 74-1 to 74-4 by the circulator 70, and the light having the same wavelength as the case shown in FIG. 9B is reflected. Then, the light is input to the circulator 71 and output to the receiving station. As shown in Fig. 9C, light of four different wavelengths is output, and the remaining light is output only as low light level noise.

10A and 10B show the input of the added optical signal to the branching unit, and show the state of the output light from the branching unit to the receiving station where the added optical signal is combined.

As shown in FIG. 10A, it is assumed that a four-wave multiplexed signal is sent from a branch station. If this signal is combined with the optical signal from the sending station without control according to conventional techniques, the result of the combination is as shown in FIG. 10B. In this example, an eight-wave multiplexed signal is sent from the sending station, and it is assumed that four shorter-wavelength optical signals (1) are added and taken out by the branching unit as shown in FIG. 10A.

If the optical signal from the branch station as shown in Figure 10A is combined with the optical signal (2) from the sending station without any control, the optical signal (2) from the sending station is combined with the optical signal (2) from the branch station as shown in Figure 10B A difference in power level occurs between the optical signals (1), because the power level of the optical signal from the branch station differs from the power level of the optical signal from the sending station. In the situation shown in FIG. 10B, the optical signal power level from the branch station is high. That is, the optical signal indicated by (1) is the optical signal from the branch station, and the optical signal indicated by (2) is the optical signal passed in the branch unit and transmitted to the receiving station. In FIG. 10A and FIG. 10B , the swollen portion is ASE noise.

11A and 11B show the case where analog light is used as the control means according to the third embodiment of the present invention.

FIG. 11A shows an optical signal containing analog light sent from a branch station to a branch unit. (4) represents an optical signal containing information. (3) represents simulated light. Comparing Fig. 11A with Fig. 10A, it can be clearly seen that if the power level of the analog light (3) is high, the power level of the optical signal containing information becomes relatively low. In the situation shown in Fig. 10B, optical signals from branch stations at higher power optical levels are combined. Therefore, there is a difference in power level between the optical signal from the branch station and the optical signal from the sending station directly to the receiving station. However, the power level of the information-containing optical signal (4) from the branch station can be reduced by means of the simulated light (3) shown in Figure 11A. Therefore, the difference in optical level between the optical signal (5) output from the sending station to the receiving station side in the branch unit and the optical signal (4) sent from the branch station as shown in FIG. 11B can be reduced to almost zero. The optical signal corresponding to (3) shown in FIG. 11B is the simulated light that is not completely dissipated in the branching unit and has been output to the receiving station.

In the above example, the power level of the optical signal containing the information sent from the branch station is relatively high. If the power level of this optical signal is low, the information-containing optical signal (4) can be made relatively high by reducing the power level of the simulated light. Therefore, by adjusting the analog optical output in the branch station according to the situation, the power level of the optical signal (5) from the sending station can become matched with the power level of the optical signal (4) from the branch station, thereby maintaining high system performance.

The above technology can be applied to optical wavelength multiplexing communication with more channels than eight-wave transmission, or to optical wavelength multiplexing communication with fewer channels. The wavelength of the analog light does not have to be shorter, but can be any wavelength as long as the wavelength is within the wavelength band used by the optical amplifier to amplify the optical signal.

Fig. 12 is a block diagram showing a part of a terminal station as a transmitting station and a receiving station.

If the optical signal is sent from the branching unit via the uplink, the coupler 90 will partially branch the optical signal. For example, the ratio of coupler 90 to optical signal branching is 10:1. Most of the optical signal passes through coupler 90 and is then branched by couplers 91 and 92 . Each branch optical signal is extracted as a signal having an individual wavelength (optical signal along an individual channel) through the optical filters 93-1 to 93-3. Optical signals having respective wavelengths are amplified by preamplifiers 94-1 to 94-3, received by optical receivers 95-1 to 95-3, and then converted into electrical signals. The demultiplexer 97 takes out information and sends it to an information processing unit not shown in FIG. 12 .

The optical signal branched by the coupler 90 is input to an optical spectrum analyzer 96, and the optical signal power level of each wavelength is checked. The optical adjustment of the received optical signal is extracted as information. Write it into the information communication format of the optical signal (for example, the POH of SDH/SONET (by overhead (pass overhead)) in the data format generation unit not drawn among Fig. 12, the electric signal that produces and other information signals put In this format.These processes are completed by the multiplexer 98 shown in Figure 12.The data signal output from the multiplexer 98 is converted by the optical transmission unit 99-1 to 99-3 on each channel Become the optical signal of corresponding wavelength, the optical signal of each wavelength utilizes post-amplifier 100-1 to 100-3 to amplify, send again.The optical signal of each wavelength that produces like this is combined by coupler 101 and 102, by downlink Transfer to end station or branch station.

The terminal station receiving the optical signal not only extracts the information in the optical signal, but also uses the spectrum analyzer 96 to detect the difference of the optical signal power level of each wavelength, and then transmits it as information inserted in part of the main signal.

In FIG. 12, the number of multiplexed optical signal wavelengths is three, but the number is not limited to three. According to FIG. 12, two of the three optical receivers 95-1 to 95-3 receive the optical signal from the branch station, and the remaining one receives the optical signal from the terminal station, that is, the sending station, and then through the branch unit. Similar to this, two of the three optical sending units 99-1 to 99-3 transmit the wavelength optical signal for transmission to the branch station, and the remaining one transmits the wavelength optical signal for transmission to the terminal station, that is, the receiving stand.

Figure 13 shows a block diagram of a portion of a branch station.

Through the downlink, two optical signals with different wavelengths (not limited to two) are sent from the branch unit. Coupler 118 branches this optical signal. Optical filters 119-1 and 119-2 extract optical signals of respective wavelengths. Optical signals of corresponding wavelengths are amplified by preamplifiers 120-1 and 120-2, and converted into electrical signals by optical receivers 121-1 and 121-2. The demultiplexer 122 extracts information, and transmits the information to an information processing unit not shown in FIG. 13 .

The demultiplexer 122 extracts the information written into the optical signal data transmission format, which is obtained by the spectrum analyzer in the terminal station (for example, the information of the difference in the received light level of each wavelength optical signal sent from the terminal station is in the POH area of SDH/SONET), and send this information to the computer 117.

The computer 117 sends the data information to be transmitted as a signal to the optical transmitters 114-1 and 114-2 to generate optical signals having respective wavelengths. In addition, the branch station includes an analog light generating unit 115 for outputting analog light. Optical signals and analog light having respective wavelengths are amplified by post amplifiers 113 - 1 to 113 - 3 , combined by couplers 111 and 112 and then transmitted. The combined optical signal is branched by coupler 110 . The light splitting ratio of the coupler 110 is, for example, 10:1, most of the light passes along the original path, and only a small part is split. The optical signal split by the coupler 110 is input to the spectrum analyzer 116, and the difference in the power level of the optical signal of each wavelength output from the branch station is detected.

The detection result of the spectrum analyzer 116 is input to the computer 117, and it is compared with the information extracted by the demultiplexer 122 about the difference in the received light level in the terminal station. A control signal simulating the optical transmission power is sent to the post amplifier 113-3. Therefore, the difference in optical level between the optical signal sent by the branch station and the optical signal sent to the receiving station can be monitored, and the optical signal sent to the receiving station is not taken out from between the terminal station and the branch unit. Adjust the transmission power of the simulated light according to the monitoring results. Therefore, the difference in power level between the optical signal emitted from the branch station and combined by the branch unit and the unextracted optical signal can be controlled so as to drop to almost zero. Therefore, the decrease of S/N ratio caused by power optical adjustment can be avoided, and high system performance can be maintained.

As shown in FIG. 13, the optical signal transmitted to the branch station uses two different wavelengths. However, the configuration of the system is not limited to this application, and the number of wavelengths of optical signals emitted from the terminal station and the number of wavelengths of optical signals transmitted to branch stations should be appropriately determined as necessary in each design step.

According to the present invention, the difference in power level between the optical signal of each wavelength emitted from the branch station in the branch unit and the optical signal of each wavelength not extracted can be compensated when the two optical signals are combined. The performance of the system can avoid being degraded by the S/N deterioration of the lower power light level. Therefore, it is possible to provide a light add-drop system maintaining high system performance.

Claims (22)

1. An optical terminal station comprising: an optical transmission signal sending device for generating an optical transmission signal modulated with data to be transmitted; an analog light generating device for generating analog light having a wavelength different from that of the optical transmission signal; a wavelength multiplexing device for wavelength multiplexing analog light and optical transmission signals; and The light level adjusting device is used for adjusting the output light level of the simulated light. 2. The optical terminal station according to claim 1, wherein The wavelength of the simulated light is in the same wavelength band as that of the optical transmission signal. 3. A method of controlling an optical communication system having an optical amplifier for maintaining a constant output signal between at least one pair of optical end stations, wherein The optical terminal station sends the optical transmission signal and an analog light having a wavelength different from the optical transmission signal, and adjusts the optical level of the analog light, thereby controlling the optical level of the optical transmission signal in the output light of the optical amplifier. 4. The method according to claim 3, wherein The wavelength of the simulated light is in the same wavelength band as that of the optical transmission signal. 5. A method of controlling an optical communication system, the system has a first optical terminal station, a second optical terminal station, a third optical terminal station, and an optical branching unit for connecting the first optical terminal station to the third optical terminal station, and an optical amplifier for maintaining a constant output signal between the optical branching unit and the second optical terminal station, causing the optical branching unit to wavelength-multiplex the optical transmission signals from the first terminal station and the second terminal station, and transmits the signal to a third terminal station, where The second optical terminal station transmits analog light having a wavelength different from that of the optical transmission signal, and adjusts the light level of the analog light to control the optical level of the optical transmission signal in the output light of the optical amplifier. 6. The method according to claim 5, wherein The wavelength of the simulated light is in the same wavelength band as that of the optical transmission signal. 7. The method according to claim 5, wherein The branching unit includes a wavelength selective filter for transmitting only the optical transmission signal to the third optical end station and preventing analog light from being transmitted to the third optical end station. 8. The method according to claim 5, wherein said third optical end station detects light level information of optical transmission signals from said first optical end station and said second optical end station, and sends this light level information to said second optical end station; and The second optical end station controls the light level of the analog light. 9. An optical terminal station having a first optical terminal station, a second optical terminal station, a third optical terminal station, an optical branching unit for connecting the first optical terminal station to the third optical terminal station, and an optical amplifier , for maintaining a constant output signal between the optical branching unit and the second optical terminal station, so that the optical branching unit wavelength multiplexes the optical transmission signal from the first terminal station and the second terminal station, and transmits the signal transmitted to the third terminal station, where The second optical end station includes: an optical transmission signal sending device for generating an optical transmission signal modulated with data to be transmitted; an analog light generating device for generating analog light having a wavelength different from that of the optical transmission signal; a wavelength multiplexing device for wavelength multiplexing analog light and optical transmission signals; and A power adjustment device for adjusting the output level of the simulated light. 10. The optical terminal station according to claim 9, wherein The branching unit includes a wavelength selective filter for transmitting only the optical transmission signal to the third optical end station and preventing analog light from being transmitted to the third optical end station. 11. An optical communication system having a first optical terminal station, a second optical terminal station, and a third optical terminal station, comprising: an optical branching unit for connecting the first optical end station to the third optical end station; and an optical amplifier placed between the branching unit and the second optical end station, wherein The second optical end station includes: means for generating an optical transmission signal modulated with data to be transmitted; Analog optical devices that generate signals at wavelengths different from those transmitted by light; and A light level adjustment device that adjusts the light level of the analog light output. 12. The system according to claim 11, wherein The branching unit includes a simulated light removing unit for removing simulated light. 13. The system according to claim 11, wherein said third optical end station detects light level information of optical transmission signals from said first optical end station and said second optical end station, and sends this light level information to said second optical end station; and The second optical end station controls the light level of the analog light. 14. The system according to claim 11, wherein The optical amplifier includes an automatic output light level control circuit for keeping the output light level constant. 15. An optical communication system comprising: A sending station, configured to send a wavelength multiplexed optical signal; a receiving station for receiving optical signals; a branch station, configured to receive an optical signal of a specific wavelength in the wavelength multiplexed optical signal, and send an optical signal of the specific wavelength; and The branching device is used to separate the optical signal with a specific wavelength from the optical signal sent by the sending station, transmit the separated signal to the branch station, and combine the optical signal sent by the branch station with the optical signal whose wavelength is different from the specific wavelength, And make the power levels of the two optical signals equal. 16. The optical communication system according to claim 15, wherein The branching device includes an optical attenuator and adjusts the power level of the optical signal from the branching station. 17. The optical communication system according to claim 15, wherein The branching device compares the power of the optical signal from the sending station with the power level of the optical signal from the branching station, and adjusts the power of the optical signal from the branching station according to the comparison result. 18. The optical communication system according to claim 17, wherein The branching device includes: an optical amplifier capable of adjusting gain; and A device that adjusts the power level of an optical signal from a branch station by changing the gain of the optical amplifier. 19. A branch unit in an optical communication system, the system includes: a sending station, used to send wavelength multiplexed optical signals; a receiving station, used to receive optical signals; a branch station, used to receive wavelength multiplexed optical signals There is an optical signal of a specific wavelength in the signal, and the optical signal of the specific wavelength is sent; and the branching unit is used to separate the optical signal with a specific wavelength from the optical signal of the sending station, and transmit the separated signal to the branch station, Combining the optical signal from the branch station with the optical signal from the sending station, and transmitting the combined signal to the receiving station, where The optical signal with a specific wavelength is split from the optical signal sent by the sending station; the split signal is transmitted to the branch station, and the optical signal sent from the branch station is combined with the optical signal whose wavelength is not a specific wavelength, And make the power levels of the two optical signals equal. 20. The branching unit according to claim 19, further comprising an optical attenuator, the attenuator being used to adjust the power level of the optical signal from the branching station. 21. The branching unit according to claim 19, wherein The power level of the optical signal from the sending station is compared with the power level of the optical signal from the branch station, and the power level of the optical signal from the branch station is adjusted according to the comparison result. 22. The branching unit according to claim 21, further comprising: an optical amplifier capable of adjusting gain; and A device for adjusting the power level of an optical signal from a branch station by changing the gain of an optical amplifier

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